Heidi Johansen-Berg, a neuroscientist at the University of Oxford, responds:

THE BRAIN is an enormously complex network of billions of neurons connected by more than 90,000 miles of fibers—long enough to traverse Russia’s coastline four times. This intricate architecture allows us to absorb information quickly and efficiently. Learning mainly takes place at synapses, the junctions between neurons where information is relayed. A synapse’s performance changes when we learn something new, obeying the principle that “cells that fire together, wire together.”

To understand this concept, first imagine trying to remember the name of a new colleague, a tall, bearded man we’ll call Joe. Your brain needs to form an association between a complex visual image and a name, which are encoded by different groups of neurons in various parts of your brain. Every time you are introduced to Joe, these sets of neurons fire simultaneously, strengthening the synaptic pathway that connects them. Next time you spot a tall, bearded man coming down the corridor, you will easily greet Joe because the visual image will be strongly linked with his name.

Many different events can increase a synapse’s strength when we learn new skills. The process that we understand best is called long-term potentiation, in which repeatedly stimulating two neurons at the same time fortifies the link between them. After a strong connection is established between these neurons, stimulating the first neuron will more likely excite the second.

In addition to making existing synapses more robust, learning causes the brain to grow larger. Optical imaging allows researchers to visualize this growth in animals. For instance, when a rat learns a difficult skill, such as reaching through a hole for a pellet of food, within minutes new protrusions, called dendritic spines, grow on the synapses in its

motor cortex, the region that allows animals to plan and execute movements.

Although we cannot see these tiny details in living human brains, we can use brain scanners to visualize larger changes that happen as we learn over longer periods. Learning to juggle, for example, increases the size of parts of the brain involved in looking at and reaching for moving objects and strengthens the pathways that connect these regions.

Are we biologically inclined to couple for life? —Chelsea Brennan, Minneapolis

Jeannine Callea Stamatakis, who is an instructor at ­several colleges in the San Francisco Bay Area, ­responds:

"Till death do us part” is a compelling idea, but with the divorce rate exceeding 50 percent, many people would very likely agree that humans have a biological impulse to be nonmonogamous. One popular theory suggests that the brain is wired to seek out as many partners as possible, a behavior observed in nature. Chimpanzees, for instance, live in promiscuous social groups where males copulate with many females, and vice versa.

But other animals are known to bond for life. Instead of living in a pack like coyotes or wolves, red foxes form a monogamous pair, share their parental and hunting duties equally, and remain a unit until death.

For humans, monogamy is not biologically ordained. According to evolutionary psychologist David M. Buss of the University of Texas at Austin, humans are in general innately inclined toward nonmonogamy. But, Buss argues, promiscuity is not a universal phenomenon; lifelong relationships can and do work for many people.